Electron gun



Jan. 12, 1960 C. K. BIRDSALL ELECTRON GUN Filed Feb. 15. 1954 Ik m BY /l TM United States Ptent ELECTRON GUN Charles K. Birdsall, iVenice, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application February 15, 1954, Serial No. 410,143

1 Claim. (Cl. 313-85) This invention relates to improved double-stream electron guns and more particularly to an electron gun for use in a double-stream wave-type microwave amplifier tube.

In a double-stream wave-type microwave amplifier tube, an electron flow is employed that comprises two electron streams, each of which has a different average velocity. In the operation of this type of tube, one of the streams is modulated with a microwave signal. The modulations of the stream are amplified by the interaction between the streams and then converted into an electromagentic output signal. Optimum amplification requires that the two streams be intermingled with each other and that the difference in the two average velocities be a predetermined value dependent on the frequency of the amplified signal. For a more complete description of the theory of operation of an amplifier tube of this type, reference is made to an article entitled, The Electron-Wave Tube, by Andrew V. Haef, Proceedings of the I.R.E. January 1949, vol. 37, No. l, pp. 4 through l0.

In general, the electron guns employed in doublestream tubes are of two types. In the first type of electron gun, two electron streams emanating from adjacent sources are directed in a collimated ow along twoY paths whence the electrons intermingle at the expense of the focusing of the two streams. An example of this type of electron gun is described in U.S. Patent 2,585,582 granted to I. R. Pierce on February 12, 1952.

In the second type of electron gun used in doublestream amplifier tubes, an intermingling of two streams is effected by projecting one electron stream through the same region occupied by another, thereby producing an electron flow constituting two streams of different average velocities traveling substantially through the same physical space. An example of this second type of electron gun is described in U.S. Patent 2,652,512, granted to A. V. Hollenberg on September 15, 1953.

The electron gun of the present invention is of the second type. In this electron gun, a first source of electrons is disposed within an inside-out cathode which provide a second source of electrons. In general, an inside-out cathode comprises a metallic cylinder with an apertured appendage substantially closing one end thereof having an electron emissive material disposed on its inner surface. Electrons emitted from the first source are directed through the aperture of the inside-out cathode constituting the second source, where they intermingle with electrons emitted from the edges of the aperture thereof, whence a confluent flow of electrons is produced.

The inside-out cathode is particularly well adapted for use in double-stream electron guns in that electron ICC this advantage is the fact that, due to the configuration of the electron emitting surface of the inside-out cathode, there is a high resistance to positive ion bombardment, which greatly lengthens its life.

It is therefore an object of the present invention to provide an improved electron gun for a double-stream microwave amplifier tube.

It is another object of the invention to provide a 'double-stream electron gun which includes at least one electron emitting surface of a configuration for producing an inherently well focused electron stream which may be readily intermingled with another.

It is still a further object of the invention to provide a double-stream wave-type amplifier tube including an electron gun incorporating an inside-out cathode to produce a well 'focused confluent flow of electrons constituting two streams. i

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanyingv drawing, in which several embodiments of the invention are illustrated by way of example.

It is to be expressly understood that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a double-stream amplifier tube together with associated circuitry wherein one em-A bodiment of the improved electron gun is shown; and

Fig. 2 is a sectionalview of an alternate embodiment of the electron gun of the present invention.

Referring to Fig. l, there is shown a double-stream amplifier tube for amplifying microwave signals com-` prising an elongated evacuated envelope 10 having an enlarged portion at the left extremity thereof, as viewed in the drawing, for housing an electron gun 12. Electron gun 12 produces a double-stream iiow of electrons which isdirected along a predetermined path 14 that lies on the longitudinal axis of elongated envelope 10.

Electron gun 12 comprises inner and outer concentric metal cylinders 16 and 18, respectively, disposed concentrically about the longitudinal axis of envelope 10 with a heating element 20 disposed therebetween. Concentric cylinders 16 and 18 have appendages facing the elongated portion of envelope 10 with electron emissive coatings 22and 24, respectively, of alkaline earth metal compounds onrthe inner surfaces thereof.' Apertures 26 and 28 are disposed in the center of the appendagesl of cylinders 16 and 18, respectively, on the longitudinaly axis of envelope 10. In. activating the cathodes, a. monatomic layer of barium is diffused over a portion of the inner surfaces of the apertures 26, 28 to produce electron emissive surfaces. Electron emission from surfaces of this configuration inherently focuses the electrons into a small stream. The electron emissive surfaces of cylinders 16 and 18 are heated to an appropriate emission takes place at the aperture edge to form an Y inherently well focused electron stream that may be readily intermingled with another stream to effect a higher degree of intermingling and interaction than possible with electron guns now Ain use. Combined with temperature to effectr thermionic emission by filament from cylinder 18. For streams of equal current densi,

ties, optimum amplification occurs at a velocity difference, which is determined by the relationship z vFao/ velocity of the stream electrons along path 14, w is the where 'l2-v1 is the velocity difference, un is the average.:

3 angular frequency of the signal to be amplified and JOC M uomceo 2) where wp is the plasma frequency of the electron streams,

J is the current density in the electron streams which is assumed to be equal in both streams e is the charge of an electron me is the mass of an electron, and

e0 is the absolute dielectric constant of free space.

An accelerating electrode 30 is disposed adjacent to the appendage of cylinder 18 perpendicular to the longitudinal axis of envelope and has an aperture 32 in alignment with apertures 26, 28. Electrode is maintained at a reference potential such as, forexample, ground, and at from 50 to 2000 volts positive with respect tothe potential of cylinder 18 by a connection from cylinder 18 to the negative terminal of a source 35, the positive terminal of d which is connected to ground. Thus, electrons emitted from the electron emissive surfaces of cylinders 16 and 18 are directed through aperture 32 along the path 14.

Disposed concentrically about path 14 in the direction of electron ilow are a matching ferrule connected over a lead 42 to an input helix 44, a conductive tubular element 46, and an output helix 48 connected over a lead 50 to a matching ferrule 52. Helices 44, 48 are connected electrically to tubular element 46 and are maintained at ground potential by a suitable connection thereto so that the average velocity of the electron stream is substantially constant over the entire length of path 14. A collector 54 for intercepting and collecting the stream electrons is disposed at the termination of path 14. Collector electrode 54 is maintained at a potential of the order of 200 volts positive with respect to the potential of the path 14 in order to prevent the return of secondary electrons to ferrule 52. This is accomplished by a connection to the positive terminal of a battery 55, the negative terminal of which is connected to ground.

An input to the tube is provided by an input waveguide 56 which symmetrically encloses a portion of envelope 10 that is coextensive with the lead 42 and has a shorted termination 58 located one-quarter of a guide wavelength therefrom in order to eect optimum coupling from the waveguide 56 to the helix 44. A sleeve 60 is disposed concentrically about envelope 10 coextensively with ferrule 40 and is electrically connected to the side of waveguide 56 nearest the electron gun 12. Both ferrule 40 and sleeve 60 are of an appropriate length to produce a virtual shorting plane on the inner surface of waveguide 56 so that substantially all of the energy is directed along helix 44 for modulating the electron streams. Waveguide 56 is preferably maintained at the same potential as that of path 14 in order not to disrupt the flow of electrons. In the present instance, this would be at ground potential.

In the operation of the tube, the input electromagnetic energy is propagated by the helix 44 along path 14 as a traveling-wave having a velocity substantially less than the velocity of light. In order that the wave progress uniformly along the path 14, it is necessary that helix 44 hold its shape, especially with respect to its pitch and diameter. Tungsten or molybdenum are metals suitable for the fabrication of the helix. Also, it is necessary to terminate the electromagnetic wave propagated by the helix 44 without reflection at the extremity thereof farthest from the electron gun 12.V This may be accomplished by a resistive coatingV 62 disposed about the last few turns of helix 44 on the outside of envelope 10.

Next along the path 14 is the conductive tubular element 46 which provides a drift region along a substantial length of the path. Element 46 may be composed of any nonmagnetic metal such as, for example, copper or nickel.

After the tubular element 46 along the path 14 is the output helix 48 which provides a means of deriving an Ioutput signalfrom the modulations of the stream. The helix 48 may be constructed in the same manner as was helix 44. An output waveguide 64 with a sleeve 66 is disposed about the envelope 10 coextensive with the lead 50 and ferrule 52, respectively, in the same manner as was done for the input waveguide 56. In that there may be reflections due to an imperfect impedance match between output helix 48 and output waveguide 64, a resistive coating 68 for terminating waves reflected at this imperfect impedance match is disposed on the outside of envelope 10 about the last few turns of helix 48 nearest tubular element 46.

A solenoid 70 is axially positioned symmetrically about the complete length of envelope 10. An appropriate direct current is maintained in solenoid 70 by means of a connection across a potential source, such as a battery 72, so as to produce a magnetic field which may be of the order of 600 to 1000 gauss running axially along the entire length of the tube. The purpose of this magnetic lield is to keep the electron stream focused or constrained while traversing the path 14 of the tube. Further, if the field is very conining, the inside-out cathodes incorporated in the present invention may produce conuent hollow streams.

In the operation of the present invention, electrons are drawn from the electron emitting portions of apertures 28 and directed along path 14 at a rst velocity determined by the voltage between the path 14 and metallic cylinder 18. As previously mentioned, the potential of path 14 is maintained constant and is determined by the voltage impressed on ferrules 40 and 52, helices 44 and 48, and tubular element 46. Further, electrons drawn from the electron emitting portions of aperture 26 are directed through aperture 28 along path 14at a second and higher velocity determined by the potential difference between path 14 and metallic cylinder 16. The electrons from aperture 26, upon passing through aperture 28, intermingle with the electrons from aperture 28 whence they all ow through substantially the same physical space that constitutes path 14. It is to be noted that the difference between the first and second velocities is controlled, in part, by the potential dilerence between metallic cylinders 16 and 18.

A microwave signal to be amplified is impressed on the input waveguide S6 whence it is coupled to helix 44 by the lead 42 which is disposed parallel to the E lield within the waveguide. The helix 44 propagates the signal as an electromagnetic wave along the path 14 at a velocity substantially less than the velocity of light. The electromagnetic Wave interacts with the electron streams in a manner that effects modulation of either one of the electron streams to produce a growing space charge wave propagated by the stream. At the termination of helix 44, the energy in the electromagnetic wave is dissipated in the resistive coating 62 and the space charge wave continues on through the drift region provided by tubular element 46. Within this region, the electrons traveling at the higher velocity interact with the electrons traveling at the slower velocity to further amplify the space charge wave. The actual degree to which the electrons traveling at the two velocities are intermingled determines the amount of the amplification of the space charge wave.

The amplied space charge wave, upon entering output helix 48, induces an electromagnetic wave on the helix which is propagated along with the stream. Substantially all of the energy of the space charge wave is transferred from the electron streams to this electromagnetic wave throughout the length of helix 48. At the termination of helix 48, remaining energy of the electron streams is dissipated at the collector electrode 54 and the electromagnetic wave `constituting the amplified signal is coupled to output waveguide 64 by means of lead 50.

A s previously mentioned, amplification in the doublestream amplifier is determined by the degree of intermingling between the high and low velocity electrons.

The improved electron gun 12 of the present invention makes a high degree of intermingling possible in that the electron stream from the emitting surface of aperture 26 may be directed through the same physical space as the electron stream from the emitting surface of aperture 28. The electron emission from the aperture 28 is from the inner surface at the edges nearest electrode 30, hence there is substantially no space charge produced to detrimentally affect the flow of electrons from cylinder 16 through the aperture 28. Further, the potential gradients produced at the edges of apertures 26, 28 tend to direct the emitted electrons inwards towards the center of the streams thus forming an inherently well focused beam.

An alternate embodiment of gun 12 is shown in Fig. 2 as electron gun 12a. In this embodiment, cylinder 16 is replaced with a solid metallic cylinder 74 having a layer of an electron emissive material disposed on the end nearest the appendage of cylinder 18. Filament 20, cylinder 18 with coating 24, and electrode 30 shown in Fig. 2 are the same as in Fig. l. The electron emissive coating 76 of metallic cylinder 74 is disposed in register with the aperture 28 defined by metal cylinder 18 thus enabling the electrons emitted from the coating 76 to be directed through the aperture 28 in the same manner as before.

In the light of the foregoing disclosure, apparatus for combining rst and second electron streams to form a confluent ow wherein the second stream is projectedv through lthe aperture of a member that provides a source of electrons for the first stream whereby the cross sectional area of the other stream overlaps at least a portion of the cross sectional area of the first stream is considered to be within the scope .of the teachings of this specification.

What is claimed is:

An electron gun for producing a double-stream ow of electrons in a wave-type amplifier tube, said electron gun comprising a first tubular metallic element having a rst appendage substantially closing one end thereof, said first appendage defining a first circular aperture at said one end of said first tubular element; a second tubular metallic element, disposed concentrically Within said first tubular element, having an appendage substantially closing the end thereof nearest said first appendage, said appendage of said second tubular element defining a second circular aperture aligned With said first aperture; a mixture of alkaline earth metal compounds disposed on the inner surfaces o-f said appendages for making the portions of the surfaces thereof that define said first and second apertures thermionically emissive; means for heating said first and second tubular elements whereby said thermionically emissive portions provide first and second sources of electrons, respectively; means for directing electrons from said first source along a path at a predetermined velocity; and means for directing electrons from said second source through said first aperture at a velocity greater than said predetermined velocity to produce a confluent flow with the electrons from said first source along said path.

References Cited in the file of this patent UNITED STATES PATENTS 2,038,341 Bruche Apr. 2l, 1936 2,201,817 Smith May 21, 1940 2,585,582 Pierce Feb. 12, 1952 2,630,547 Dodds Mar. 3, 1953 2,652,513 Hollenberg Sept. 15, 1953 2,684,453 Hansell Iuly 20, 1954 2,810,088 MacNair Oct. 15, 1957 2,862,128 Robertson Nov. 25, 1958 

